1. Field of the Invention
The present invention relates to apparatus for producing interference patterns commonly encountered in optical metrology. More particularly, the invention relates to apparatus for use in conjunction with either plano or spherical Fizeau or Twyman-Green type interferometers for the purpose of rapidly and accurately measuring the distortion of surfaces or transmitted wavefronts. No physical contact with the article under test is required.
2. The Prior Art
Interferometric testing has long been used in optical metrology. The advent of the laser has not only made interferometers more convenient to use but has also extended their range of application. Interferometry is used as a tool in fabrication, final testing, and system alignment, see for example, C. Zanoni, "Interferometry," The Optical Industry and Systems Directory Encyclopedia, v. 2, E137 (1977).
With the increased use of optical interferometers, there has been a corresponding need to provide the user with increased flexibility and versatility. For example, it is desirable to be able to conveniently and economically measure articles over a broad range of aperture sizes, i.e., typically from 16 mm to 100 mm diameter. Furthermore, it is imperative that the interferometer cavity be easily and quickly aligned to produce interference patterns. In addition, because of DHEW requirements on laser product viewscreens and the increasing use of video interference pattern evaluation apparatus, it is important for an interferometer system to have an integral CCTV camera to allow viewing the alignment spots and the interference pattern on a CCTV monitor.
Since Fresnel diffraction distorts the fringes at the edge of the aperture and since this phenomenon is particularly evident when small apertures are measured, it is important for an interferometer system to incorporate aperture focusing.
In many industrial applications, it is desirable to produce interference patterns with an interferometer system with the aforementioned capabilities. Prior art systems lack one or more of the above mentioned desirable features.